TNF alpha increases activity of gamma-glutamyl transpeptidase in cultured rat astroglial cells

To investigate the presence of gamma-glutamyl transpeptidase (gammaGT) in brain cells, cultures enriched for astroglial cells, neurons, oligodendroglial cells, and microglial cells were studied. Astroglial cultures contained a specific gammaGT activity of 2.3 +/- 0.9 nmol/min/mg protein. A similar specific gammaGT activity was measured for oligodendroglial cultures, whereas microglial cells and neurons contained less than 30% of the specific gammaGT activity of astroglial cultures. The activity of gammaGT in astroglial cultures was elevated strongly by the presence of tumor necrosis factor-alpha (TNFalpha) in a time- and concentration-dependent manner. Maximal activity of gammaGT was observed after incubation of astroglial cultures for 3 days with 30 ng/mL TNFalpha. Under these conditions the specific gammaGT activity was increased by threefold compared to controls. Presence of the gammaGT-inhibitor acivicin completely inhibited gammaGT activity both in TNFalpha-treated and in control cells. In addition, the increase in astroglial gammaGT activity after application of TNFalpha was prevented completely by the presence of the protein synthesis inhibitor cycloheximide. gammaGT is involved in extracellular processing of glutathione (GSH) that is exported by astroglial cells. After TNFalpha-treatment the concentration of GSH in the medium of astroglial cells was reduced significantly compared to control cells. In conclusion, the data presented demonstrate that TNFalpha stimulates gammaGT synthesis in astroglial cells and thereby improves the capacity to process GSH exported by these cells.